lang F1 6: contextually type pack-fn prefix args (mapper lambda)

lowerPackFnCall lowered the runtime prefix args with no target_type, so a
lambda arg (mapper: Closure(...) -> ...) could not infer its param types.
Now set target_type to the param type while lowering each prefix arg. With
the existing value-projection call-arg spread, mapper(..sources.get) works:
the lambda is contextually typed and the projected values spread into the
call. examples/211 ((a,b)=>a+b over two sources -> 42). 246 + unit green.

examples/211-mapper-projection-spread.sx

@@ -0,0 +1,18 @@
+// Phase 6 — `mapper(..sources.value)`: project a method over a pack and spread
+// the results into a closure call. The mapper lambda's params are contextually
+// typed from the `Closure(...)` parameter even though `apply` is a pack-fn.
+
+#import "modules/std.sx";
+
+VL :: protocol(T: Type) { get :: () -> T; }
+IntCell :: struct { v: s64; }
+impl VL(s64) for IntCell { get :: (self: *IntCell) -> s64 => self.v; }
+
+apply :: (mapper: Closure(s64, s64) -> s64, ..sources: VL) -> s64 {
+    return mapper(..sources.get);     // (a, b) => a + b  applied to (s0.get(), s1.get())
+}
+
+main :: () -> s32 {
+    print("{}\n", apply((a, b) => a + b, IntCell.{ v = 40 }, IntCell.{ v = 2 }));  // 42
+    0;
+}

src/ir/lower.zig

@@ -9195,7 +9195,13 @@ pub const Lowering = struct {
                 if (isPackParam(p)) break;
                 if (ri >= call_node.args.len) break;
                 if (!p.is_comptime) {
+                    // Contextually type the arg from the param (so a lambda arg
+                    // `(x) => …` takes its param types from a `Closure(...)` param).
+                    const saved_tt = self.target_type;
+                    const pty = self.resolveParamType(&p);
+                    if (pty != .unresolved) self.target_type = pty;
                     args.append(self.alloc, self.lowerExpr(call_node.args[ri])) catch return self.builder.constInt(0, .void);
+                    self.target_type = saved_tt;
                 }
                 ri += 1;
             }

tests/expected/211-mapper-projection-spread.exit

@@ -0,0 +1 @@
+0

tests/expected/211-mapper-projection-spread.txt

@@ -0,0 +1 @@
+42